Many older adults have difficulty understanding speech in complex and demanding environments typical of everyday listening. Using laboratory outcome measures, speech-understanding difficulties of older hearingimpaired adults are accounted for primarily by reductions in speech audibility due to elevated thresholds. Given the importance of speech audibility, amplification provided by a hearing aid should restore important speech information and provide significant benefit to communication. Nevertheless, only a small fraction of older adults who could benefit from amplification own hearing aids and use them regularly. The lack of success and satisfaction with hearing aids suggests that a fundamental change in direction is needed. The design and fitting of communication aids for older adults should be guided by their listening performance in complex and demanding environments that contain the acoustic and perceptual variability inherent in everyday situations. Thus, in Project 1, a series of experiments is planned to address key questions concerning older adults' perception of speech information as delivered in realistic environments. To relate age-related declines in auditory function to underlying mechanisms of age-related hearing loss, subjects will include older adults with and without characteristics of metabolic presbyacusis. Aim 1.1 tests the hypothesis that, even when audible, limited use of interaural level and timing cues reduces the binaural advantage for speech for older adults with and without hearing loss and that binaural advantages vary according to the phenotype of age-related hearing loss. Aim 1.2 tests the hypothesis that benefit of hearing aids for processing spatial, binaural, and temporal information underlies hearing-aid use and success by older adults and differs according to the phenotype of age-related hearing loss. Three hearing-aid trials are conducted to determine the extent to which adults with and without characteristics of metabolic presbyacusis who are predicted by laboratory-based measures to benefit from amplification ultimately benefit from using hearing aids in everyday communication. Aim 1.3 tests the hypothesis that age-related changes in speech audibility and in auditory attention increase the effects of stimulus similarity and uncertainty, especially in adverse listening conditions. A long-term goal is to extend our knowledge of mechanisms that account for reduced speech understanding in noise by older adults and thus enhance benefit derived from amplification.